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I know that you can easily determine the percent capacity remaining in a battery using an arduino by dividing the voltage down to something that the ADC can read. What I'm stuck on is how the voltage will vary as the battery discharges. I've looked at multiple datasheets and tried to look on Duracell's website for one (I'm using a Duracell Ultra SLA battery), but I couldn't figure out how the voltage behaves as the battery discharges.

The main thing I need to figure out is the voltage at which the battery is considered "dead." But I think I also need to figure out how to read the voltage curves in the datasheets because (from what I can tell) the voltage does not vary linearly with the battery's capacity remaining. Can anyone point me in the right direction for information about this?

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  • \$\begingroup\$ If this will be used for a single application you could create a battery curve (or discharge profile) by using a fixed load and measuring the voltage regularly. You'll then have a rough idea of the expected voltage at each point. You could also use a coulomb counter and forget about voltage altogether. \$\endgroup\$ – David Jul 25 '17 at 22:47
  • \$\begingroup\$ @David The application will be a solar-powered project, so the battery will have a variable amount of charging and load at any given time, so I don't think I can use a coulomb counter. As for the battery curve, what kind of fixed load could I use for that? Would a motor work? \$\endgroup\$ – Anthony C Jul 25 '17 at 22:51
  • \$\begingroup\$ Anthony, be prepared that by measuring the battery voltage, you will be able to estimate the state of charge within about 25%, but not more accurately. Battery voltage is not an accurate method for estimating state of charge, although it's better than nothing. \$\endgroup\$ – Nick Alexeev Jul 26 '17 at 2:00
  • \$\begingroup\$ @NickAlexeev Yeah I've heard that open circuit voltage is not the best method, but I'm not concerned about having a ton of accuracy. I just want some idea of how fast the battery is discharging and if the solar panel is keeping up. \$\endgroup\$ – Anthony C Jul 26 '17 at 2:27
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The "dead" or "end" voltage you are looking for is 11V (1.83V/cell) at room temperature.

Below are typical discharge curves for a 20Ah 12V battery - note how the 1A curve ends just below 11V. The higher current curves go below 11V because they consider the additional voltage drop due to the internal resistance of the battery.

enter image description here

There are two related voltages worth knowing: "bulk" and "trickle" - this article is a brief but excellent introduction. Your manufacturer's data sheets generally provide the most pertinent information for your specific battery.

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